Cunningham Equation Calculator
Estimate resting metabolic rate from fat-free mass with the Cunningham (1991) equation, RMR = 370 + 21.6 × FFM — the same constants used by Katch-McArdle. Enter weight and body fat %.
Cunningham Equation Calculator
Not used in the formula — it only sets the minimum healthy body-fat floor.
Range: 30–300 kg · up to 2 decimals
Optional — used only to apply teen-safety guidance. It does not change your BMR.
Range: 3–45% · up to 2 decimals
Not sure? See typical body fat ranges
Men: athletic 6–13% · fit 14–17% · average 18–24% · higher 25%+
Women: athletic 14–20% · fit 21–24% · average 25–31% · higher 32%+
A rough estimate still gives a usable number — for a precise reading, use the Body Fat Calculator below.
Add your height (optional — enables an underweight safety check)
Enter your height in centimeters (e.g., 170 cm)
Not used in the Katch-McArdle BMR — only to check your weight isn't already in the underweight range before showing fat-loss targets.
*This calculator is for informational purposes only. Please consult a healthcare professional before making any health decisions. See our medical disclaimer for more information.
Quick Answer
The Cunningham Equation
The Cunningham equation (1991) estimates resting metabolic rate from fat-free mass: RMR = 370 + 21.6 × fat-free mass (kg). These are the exact constants the Katch-McArdle formula uses — the two are the same maths under different names. This calculator takes your weight and body fat %, works out your fat-free mass, and returns your RMR and daily calories. Enter your numbers above to run it.
The Equation, and a Worked Number
The Cunningham equation is a single line: RMR = 370 + 21.6 × fat-free mass in kilograms. Fat-free mass is simply your weight minus your fat mass, so if you enter weight and body fat percentage the tool derives it as weight × (1 − body fat% ÷ 100). Work an example for someone with 70 kg of fat-free mass: 21.6 × 70 = 1,512, and 370 + 1,512 comes to about 1,882 kcal a day. That is the resting figure; multiplying by an activity factor gives total daily energy expenditure, exactly as the Katch-McArdle result above does.
1980 vs 1991: Which Constants Are These?
It is worth being precise about the coefficients, because two versions circulate. Cunningham's earlier 1980 work gave RMR = 500 + 22 × fat-free mass; his 1991 re-analysis produced the lower 370 + 21.6 × fat-free mass, and it is the 1991 form that Katch-McArdle uses and that this page runs. The difference is not cosmetic: for 70 kg of fat-free mass the 1980 equation returns about 2,040 kcal against the 1991 equation's 1,882 kcal — roughly 160 calories apart. If a calculator elsewhere shows a noticeably higher number, check whether it is quietly using the older 500 + 22 constants.
Getting an Accurate Fat-Free Mass
The equation is only as good as the fat-free mass you feed it, so that is the number worth getting right. If you have a measured body fat percentage, the tool computes fat-free mass exactly; if you don't, the Lean Body Mass Calculator estimates it from height, weight and sex using the Boer, James and Hume formulas. Once you have your resting figure, apply an activity factor to reach maintenance — the TDEE Calculator does that step and lets you compare Cunningham/Katch-McArdle against the size-based formulas side by side.
Source: Cunningham JJ. Am J Clin Nutr. 1991;54(6):963–969 (the 370 + 21.6 form); the earlier 500 + 22 form is from Cunningham JJ, 1980. These figures are general estimates for education, not medical advice.
Precision Tracking Tools
Monitor your baseline metrics with clinical accuracy to ensure you stay on target.
Custom Meal Design
Access foundational dietary blueprints structured for sustainable, long-term success.
Expert Coaching
Connect with certified professionals who can tailor strategies to your unique physiology.
Explore More Tools
Each calculator feeds into the next — your data carries forward automatically through every step.
Katch-McArdle
Get the most accurate BMR and TDEE for lean, muscular bodies — calculated from your lean body mass and body fat %.
Lean Body Mass
Find your lean mass and fat mass from height, weight and sex (Boer, James, Hume) — or directly from a measured body fat %.
TDEE
Find out exactly how many calories your body burns daily — the foundation of every weight loss plan.